The object of this study is to investigate the possibility that changes in the blood volume within a region treated by radiotherapy may be used as an early indicator of response to therapy. A preliminary study in a canine with osteosarcoma carried out in this laboratory demonstrated that large changes in blood volume take place as a result of radiation therapy treatment. Vascular volume increased by 40% at 24 hours following the beginning of a 2 week course of radiotherapy and then decreased to 1/2 the pretreatment blood volume by day 39. The methodology involved requires the patient to breathe air containing a trace quantity (less than 0.04 ml CO total) of carbon-11 labeled carbon monoxide (10 mCi 11CO total) and about 5 minutes later to have the activity distribution within the region of interest imaged from outside the patient using a body scanning system capable of absolute activity quantitation. This data is then processed giving a blood volume map of the tumor region and control contralateral or other standard body position. This proposal will restrict itself to following lung tumor response to therapy with attempts to correlate early blood volume changes with the later observation by radiographic appearance of (1) radiation pneumonitis, (2) fibrosis and (3) tumor regrowth. It may prove possible to predict the intensity of these tissue injury effects at an earlier time by observation of the intensity of the early vascular volume changes. Later increases in blood volume may prove a sensitive indicator of tumor regrowth.